Currency of purchase is not relevant; if you buy 10 bananas for $1 US or $1.3 CAD, you still own 10 bananas. An ETF tracking an index should perform the same after fees whether it is Canadian or US (this isn't 100% true, but you're generally going to be better off with the US stuff). Because of the market sizes, I don't think you'll find US-domiciled stuff listed in Canadian money, so it's moot.
You want, I believe, to own US domiciled ETFs; this will mean they are in USD. So, VT, VXUS, etc.
The reason to use Questrade is that you will get Canadian tax slips. I think they will also send you US info. Where a US account will not give you the Canadian info (T slip), you'll have to work it all out yourself... though there is an argument that says you should do this anyway as the values given can be wrong (with respect to return of capital generally, I think).
If you have more than $100k CAD in foreign property you must file T1135. It is a massive pain, but there are penalties to not doing so. Just be aware.